1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gas which entraps platinum into a complex oxide and comprises a complex oxide having a good heat-resistance performance even in a lean combustion atmosphere at a high temperature of not lower than 1000.degree. C.
2. Description of the Prior Art
A catalyst for purifying an exhaust gas for automobiles, etc, according to the prior art uses a precious metal such as platinum, palladium and rhodium, either alone or in combination, as a catalytic component, and the catalytic component is generally supported by a catalytic support. Among them, however, rhodium is not abundant and is expensive. Palladium has better heat resistance than platinum but has remarkably lower resistance to poisoning by the lead in gasoline, or to phosphorus in a lubricating oil than platinum. Therefore, platinum which is more plentiful than rhodium becomes an essential component.
However, platinum causes the problems in that it is oxidized in a high temperature lean atmosphere, and that its surface area drops due to sintering and its activity as a catalytic component then drops remarkably.
Furthermore, an improvement in catalysts, for better heat resistance, has been required because the exhaust temperature has become higher so as to cope with stricter exhaust gas regulations such as the European step III regulations and the .lambda.=1 regulations. For existing Pt/Al.sub.2 O.sub.3 system catalysts, purification performance drops remarkably in a high temperature lean atmosphere, and they cannot satisfy such regulations. It is assumed that the drop of purification performance, too, results from sintering of platinum.
As a well-known technology in this field, the applicant of the present invention proposed a catalyst for purifying an exhaust gas which can prevent thermal degradation of Pt and its alloying and can improve the durability as well as the purification performance by using a perovskite type complex oxide between Pt and a lanthanoid element, or an alkaline earth metal element or its analogous complex oxide, as a catalyst for purifying an exhaust gas for an internal combustion engine, in Japanese Unexamined Patent Publication (Kokai) No. 62-277150. Although capable of drastically improving durability, etc, in comparison with the conventional catalysts, this catalyst is not free from the problem that the perovskite type complex oxide starts being decomposed in a region where the exhaust gas temperature exceeds 900.degree. C. As described above, the exhaust gas temperature has drastically risen due to various kinds of exhaust gas regulations in recent years, and the development of a catalyst which can sufficiently purify the exhaust gas even in a region higher than 1,000.degree. C. is required.